1. Field of the Invention
The present invention is related to the field of logging wellbores with electric wireline tools. More specifically, the present invention is related to the use of acoustic pulse-echo imaging tools in wells which have been cased and cemented. Such tools are run on electric wireline, cable, tubing, or pipe in wellbores to ascertain data, including acoustic reflection properties and acoustic travel times, from which information on geologic formations surrounding the wellbores may be obtained.
2. Description of the Related Art
Acoustic pulse-echo imaging tools are known in the art. For example, "The Digital Circumferential Borehole Imaging Log--CBIL.TM., " Atlas Wireline Services, Houston, Tex. (1993), describes an acoustic pulse-echo imaging tool in detail. The acoustic pulse-echo imaging tool usually comprises a rotating head on which is mounted an acoustic transducer, such as a piezoelectric or bender-type transducer. The transducer periodically emits an acoustic energy pulse on command from a controller circuit in the tool. After emission of the acoustic energy pulse, the transducer can be connected to a receiving circuit, generally located in the tool, for measuring a returning echo of the previously emitted acoustic pulse Which is reflected off the wellbore wall. Circuitry, which can be in the tool or at the earth's surface, measures the echo or reflection travel time and the reflection amplitude. The measurements of reflection time and reflection amplitude are used by circuitry at the earth's surface to generate graphs or images which correspond to the visual appearance, structure or other properties of the wellbore wall. The graph is used, for example, to measure the attitude of sedimentary features and to locate breaks or fractures in some earth formations in wellbores without casing. Measurements from acoustic pulse-echo imaging tools may also be used to derive the thickness of casing in wellbores with casing, as discussed in U.S. Pat. No. 5,491,668, issued Feb. 13, 1996, to John F. Priest and assigned to Western Atlas International, Inc.
Generally, however, logging behind casing, and ascertaining the type of information about a formation that is available from logging uncased wells, has not been available for wells with casing. Casing is installed on at least a portion of most completed wellbores and is most commonly used for hydraulically isolating an earth formation that could be damaged or contaminated by fluids which may be produced from a different earth formation penetrated by the wellbore. A fill material, typically referred to as, and comprising, concrete or cement, installed in the annulus portion of the wellbore between the casing and the earth formation, further prevents migration of fluids from one zone to another, and is particularly useful in separating oil and gas producing layers from each other and from water bearing strata.
Removing casing and concrete for logging is avoided because doing so may damage the well and, if the well is in production, requires taking the well out of production and risking the need of a workover of the well prior to resuming production again. Unfortunately, a large number of wells were drilled and cased worldwide prior to the advent of high resolution imaging tools, such as acoustic imaging systems, and thus information that these tools can detect about a formation does not exist for those wells. Of particular interest are thin bed sequences of sands and shales or fractures which were generally not detected by earlier instruments. Such sequences may show the presence of producing zones which were missed by the earlier technology wellbore test probes. However, drilling new test wells is costly, and may not result in sufficient new production to pay for the cost of the drilling. There is accordingly a need for logging behind casing and behind casing imaging.
Most currently known work in behind casing acoustics has been at long wavelengths, which cannot adequately describe the position, depth and character of features in the vicinity of the wellbore. Little work has been done in trying to use high frequency signals in behind casing logging, because of the improbability of recovering usable signals. High frequency signals generate massive amounts of data, and multiple reflecting surfaces in turn generate multiple reflections. The prospects of obtaining usable signals in such a situation were apparently considered to be too improbable to merit serious consideration.
U.S. Pat. No. 5,274,604, issued Dec. 28, 1993 to Ralph M. D'Angelo, et al. and assigned to Schlumberger Technology Corp., reports that, in general, any acoustic measurement used on a casing-cement-formation system will result in received energy which is dominated by casing reflections. That patent describes a method for partially filtering signals representing formation and channel echoes in a borehole environment to characterize channels formed between the material just outside the casing and the material beyond that. The filtered signals are said to indicate the quality of isolation achieved by the fill material or cement outside the casing and to provide a measurement of the thickness of the fill material. No suggestion is made of trying to find or use any signals to log formations behind casing.
A need accordingly still exists for a method of logging formations behind casing to ascertain features of the formations and particularly to identify narrow producing zones that were missed prior to installation of casing.